This invention pertains to a process for cracking and hydroconversion of heavy hydrocarbon feedstocks such as crude or residual oils to produce lighter hydrocarbon liquids such as naphtha and distillates and fuel gas products. It pertains particularly to such a process and reactor apparatus utilizing multiple zones containing fluidized beds of particulate carrier material to facilitate cracking the feedstock in an upper zone and gasification of coke deposits on the carrier in a lower zone.
Considerable work has previously been done for the multi-zoned conversion of heavy oil feedstocks using a circulated particulate carrier material. A typical process utilizes a three-zone reactor having an upper zone for primary cracking, an intermediate zone for stripping/secondary cracking and a lower zone for combustion/gasification, with each zone containing a fluidized bed of particulate carrier material which is contiguous from zone to zone. The feedstock is first cracked on and within the particulate carrier material in the upper zone and carbon is deposited on and within the carrier, after which the carbon-laden particulates descend through the stripping zone countercurrent to a rising flow of hot reducing gas. The carrier material is regenerated by partial oxidation of the carbonaceous material in the gasification zone, and is recycled by a transport gas in a riser conduit into the primary cracking zone to provide the heat of reaction therein. Some typical pertinent patents include U.S. Pat. Nos. 2,861,943 to Finneran, 2,885,342 to Keith, and 2,885,343 to Woebcke, which disclose the use of a circulating particulate carrier for coke laydown from cracking crude and residual oil feedstocks. Also, U.S. Pat. Nos. 2,875,150 to Schuman and 3,202,603 to Keith et al disclose multi-bed hydrocracking and conversion processes for residual oils and tar feeds using a particulate carrier material for hydrocracking the heavy oil feed to produce gas and liquid fractions.
In such a conversion process for heavy hydrocarbon feedstocks, it is desirable to maintain a large temperature gradient across the fluidized bed stripping zone separating the primary cracking and gasification zones. However, such a temperature gradient is difficult to achieve in a stable dense phase fluidization regime. Poor gas-solids contact between the stripping and gasification zones can limit secondary cracking temperatures achieved in the stripping zone. Mechanical design of the fluidized bed stripping zone must account for it being adjacent to the gasification zone, which is at the preferred temperatures of 1600.degree.-1900.degree. F. Also, control of the recirculating flow of hot decoked carrier solids requires throttling through a hot valve, thus contributing to mechanical design complexity.
The hydrocarbon conversion process and apparatus of the present invention provides an improvement over prior art hydrocracking processes, by providing an interim zone located between the stripping zone and lower gasification zone and arranged for achieving improved control of temperature, carrier solids flow and secondary cracking reactions in that region.